The present invention relates to a service/parking brake system especially useful in medium to heavy duty vehicles.
FIG. 1 illustrates one type of a prior art service/parking brake often called an in-wheel parking brake. There is shown a drum brake 10 comprising a drum 12 having a plurality of brake shoes 14a and 14b. The left hand side of each brake shoe 14a and b is attached to a wheel cylinder/adjuster assembly 16a and 16b respectively. The gap between friction material generally shown as 18 attached to each brake shoe and move the shoes outwardly in response to hydraulic pressure. The right hand end of each brake shoe 14a and 14b is attached to a hydraulic wheel cylinder generally shown as 20. As is known in the art, this type of hydraulic wheel cylinder includes a first piston 22 and a composite piston 24 having an upper portion 26a and lower portion 26b. During normal service, brake operation hydraulic fluid is received from a master cylinder in the space 28 between the pistons 22 and 24. In response to the actuation of the brake pedal these pistons 22 and 24 are separated thereby urging the friction material 18 into the drum 12. The hydraulic cylinder 20 in cooperation with a parking brake expander 30, is used to achieve a parking brake function. The parking brake expander 30 includes a housing 32 defining therein a fluid chamber 34. Situated in the housing 32 is a piston 36 biased outwardly by a spring 38. The chamber 34 is connected to a source of fluid such as the power steering pump. Lengthy fluid-carrying lines interconnect the power steering pump to the parking brake expander 30. Depending upon the length of the vehicle underbody, these lines can be 15 to 25 feet long as they typically extend from the engine compartment to the rear axle wheels where the parking brake expander 30 is located. During normal operation, fluid from the power steering pump enters the chamber 34 thereby urging the piston 36 outwardly from a cavity 40 located within the lower piston 24. With the parking brake piston 36 operatively removed from the hydraulic piston 20, the brake 10 is operated as a normal service brake. When the engine is turned off or a parking brake switch is actuated, pressurized power steering fluid is no longer able to operate against the force of the spring 38, thereby permitting such spring to urge the piston 36, and in particular a piston or wedge 42 or other element as is known in the art, into the chamber 40 thereby urging the piston portions 26a and 26b outwardly to activate the brake 10. As a result of the motion of the piston 36 of the parking brake expander, the brake shoes are urged outwardly, thereby achieving a parking brake function within the service brake.
The type of system described above is costly in that it utilizes a dual fluid system, that is, hydraulic brake fluid to actuate the service brake and power steering fluid to activate the expander piston 36. As mentioned above, lengthy fluid lines are needed to communicate the power steering pump to the parking brake expander 30.
An object of the present invention is to provide a braking system which is electro-hydraulically actuated and more specifically to provide an electro-hydraulically operated service brake system. A further object of the present invention is to provide a service/parking brake system which utilizes a single type of fluid, namely brake fluid.
Accordingly the invention comprises: a system for controlling service brake and/or parking functions of at least one wheel of a vehicle comprising electro-hydraulic means for providing the primary means of control of the service brake and parking brake functions. The means includes an electric motor drivingly connected to a hydraulic pump for pressurizing hydraulic fluid. The output of the pump is communicated to at least one service brake. The service brake includes a first hydraulic cylinder. First means are provided for controlling the speed of the motor to regulate hydraulic pressure in the first hydraulic cylinder in response to operator braking activity. As will be seen, the motor and pump provide the primary means of service brake activation. The output of the pump is also communicated to a parking brake mechanism, comprising a second hydraulic cylinder. The parking brake mechanism has an activated mode of operation corresponding when the second hydraulic cylinder is unpressurized and a deactivated mode of operation corresponding when the second hydraulic cylinder is pressurized. The system includes means for repressurizing the parking brake mechanism when a reduction in its stored pressure is sensed.